This invention relates to the detection of a voltage across a load in an apparatus using an electric discharge, such as a pulsed arc welding device, a laser device or a surface treating device, and to a device for most suitably controlling the output conditions of the above-described machine and devices by using the voltage detection signal. More particularly, the invention relates to a device for calculating a concerning load voltage such as a discharge voltage from a voltage value containing the reactance and resistance of a current cable, and to means for suitably controlling the above-described machine and devices according to the detected load voltage.
Typical examples of the device of this type operating on electric discharge are an arc welding machine, a laser device employing electric discharge for its exciting source, and a surface treading device utilizing glow discharge. A load voltage detecting device in a pulsed arc welding machine, which is one example of the arc welding machine, will be described.
In a pulsed arc welding machine, a pulsed arc current is allowed to flow between a consumable welding wire electrode (hereinafter referred to merely as "a wire electrode", when applicable) and a material to be welded (or base metal) to cause pulsed arc discharge therebetween. In this operation, the pulsed arc discharge generates heat, which melts the base metal and a filler metal, and the filler metal thus molten is transferred onto the part to be welded by the electromagnetic pinching force of the pulsed arc discharge.
The pulsed arc welding machine will be described in more detail. FIG. 13 shows the arrangement of a conventional pulsed arc welding machine which has been disclosed for instance by Published Unexamined Japanese Patent Application No. 19177/1982. In FIG. 13, reference numeral 1 designates the pulsed arc welding machine; and 2, an arc load section. The pulsed arc welding machine comprises: a pulse current supplying section 3 including DC source 3a, a switching element 3b for chopping current, which is a power transistor element which switches on and off current from the DC source 3a to form a pulse-shaped current waveform, and an arc maintaining power source 3c for supplying a continuous base current to the switching element 3b in order to prevent the difficulty that arcing is discontinued between pulses; a control circuit 5 for controlling the switching element 3b to set the pulse frequency and pulse width of the pulse current to predetermined values; a voltage detector 6 for detecting an output voltage V; a current detector 7 for detecting a current i; a subtractor 8 for subjecting the output voltage V and the output voltage Vo of an arc length setting unit 9 to subtraction and applying the result of subtraction to the control circuit 5; and the arc length setting unit 9.
The arc load section 2 comprises: a welding torch 2a; a wire electrode 2c which is a filler metal in the form of a wire wound on a wire reel; and a material to be welded (or base metal) 2b. A pulsed arc current from the pulsed arc welding machine 1 is supplied through cables 4a and 4b to the arc load section 2.
The operation of the pulsed arc welding machine will be described. In general, with a pulsed arc welding machine, unlike a DC arc welding machine, even when the average current is small, the end portion of the wire electrode 2c is molten by the pulsed arc current, and the end portion thus molten is caused to come off in the form of a molten drop by the electromagnetic pinch force of the pulsed arc current. The molten drops thus formed are intermittently transferred onto the base metal 2b (the transferring being referred to as "spray transfer"), thus achieving the welding operation. Accordingly, the pulsed arc welding machine can perform a welding operation with an average current lower than that in the DC arc welding machine; that is, it can perform the spray transfer welding of a base metal thinner. Thus, the pulsed arc welding machine is advantageous in that the spray transfer welding eliminates the sputtering phenomenon which otherwise may be caused during welding. In a welding operation with the pulsed arc current waveform, arc length control is carried out to prevent the change of an arc length which may be caused by the variation of the welding torch 2a, distortion of the base metal 2b, and the variation of the wire feeding speed. In the arc length control, in general, the output voltage of the pulsed arc welding machine 1 is detected by the voltage detector 6, and the output voltage V thus detected is compared with the output signal Vo of the arc length setting unit 9, and according to the difference signal (Vo-V) therebetween, the pulse width .tau. or pulse period C.sub.B is controlled (increased or decreased) thereby to suppress the variation of arc length. More specifically, when the output voltage V is larger than the output signal Vo, then it is determined that the arc length is longer than that in the steady operation, and the pulse width .tau. is decreased or the pulse period C.sub.B is increased to decrease the amount of melting of the wire electrode, under the condition that the amount of control is in proportion to the absolute value of the difference signal (Vo-V). When, in contrast, output voltage V is smaller than the signal Vo, then it is determined that the arc length is shorter, and the pulse width .tau. is increased or the pulse period C.sub.B is decreased under the condition that, similarly as in the above-described case, the amount of control is proportional to the absolute value of the difference signal (Vo-V).
In the pulsed arc welding machine thus constructed, the output voltage V detected by the voltage detector 6 is the sum of the arc voltage Va and the voltage drops VL and Vr due to the impedance of the cables 4a and 4b and the welding torch section 2a; i.e., the reactor component L and the resistance component r thereof (V-Va+L.multidot.di/dt+ri); that is, the output voltage V is not the true arc voltage Va. Therefore, as the frequency of the pulse current waveform increases, L.multidot.di/dt corresponding to the reactance voltage V.sub.L is increased os that V.sub.L occupies a larger part of the output voltage V. As a result, it becomes difficult for the output voltage to suppress the variation of the arc length which may be caused by the variation of the welding torch, the distortion of the base metal, etc.; that is, it becomes difficult to perform the arc length control stably, and accordingly the welding performance is greatly lowered. In addition, the reactor component L and the resistance component r, depending on the length of the cables, affect V.sub.L (=L.multidot.di/dt) and Vr (=ri).